The flue gas generated in the pressurized furnace, which is in the form of a fluidized furnace, of a known combination gas-turbine and steam-turbine plant (German OS 3 204 672) enters the gas-turbine combustion chamber at the temperature of the turbulent bed. Inside the combustion chamber the temperature of the gas is elevated by burning the combustion gas generated in the gasification reactor. The combustion air entering the gasification reactor and gas turbine is preheated in flue surfaces accommodated in the furnace's fluidized bed. Additional flue surfaces in the fluidized bed and gasification reactor communicate with a steam-circulation system. Some of the released heat is accordingly forwarded to the steam-turbine component of the process, which is less efficient than the overall combination of gas turbine and downstream steam turbine.
In a strictly gas-turbine and steam-turbine combination process, the total heat of the fuel is liberated in the gas-turbine combustion chamber. Enough extra air is supplied to ensure the permissible gas-turbine intake temperature. Only the waste heat downstream of the gas turbine is transmitted to the steam system in a waste-heat boiler. The gas turbine is accordingly responsible for 50% or more of the block's total output. It has been possible to attain the currently employed gas-turbine gas-intake temperatures of over 1100.degree. C. only with natural-gas and oil furnaces.
Also known are combination gas-turbine and steam-turbine plants with fluidized furnaces that have only air-cooled suspended flue surfaces for the heat of combustion to be transmitted to (VBG Kraftwerkstechnik 59 [1979], 634-40 and VDI Berichte 715 [1989], 183-84). The flue gas and heated air is supplied to the gas turbine at a temperature slightly above the combustion temperature of approximately 850.degree. C. This air-cooled circulation system is considered less economical than a steam-circulation system due to the limitation on the gas temperature upstream of the gas turbine.